The present invention relates to a method of producing a phase mask for processing an optical fiber and also relates to an optical fiber with a Bragg diffraction grating fabricated by using the optical fiber-processing phase mask. More particularly, the present invention relates to a method of producing a phase mask for fabricating a diffraction grating by using an ultraviolet laser beam in an optical fiber used for optical communications or the like. The present invention also relates to an optical fiber with a Bragg diffraction grating fabricated by using the mask.
Optical fibers have brought about a great revolution in global communications and allowed high-quality and large-capacity transoceanic telecommunications. It has heretofore been known that a Bragg diffraction grating is fabricated in an optical fiber by producing a periodic index profile in the core along the optical fiber, and the level of reflectivity of the diffraction grating and the width of the wavelength characteristics of the diffraction grating are determined by the period and length of the diffraction grating and the magnitude of refractive index modulation thereof, thereby allowing the diffraction grating to be used as a wavelength-division multiplexer for optical communications, a narrow-band high-reflecting mirror for use in a laser or a sensor, a wavelength selection filter for removing extra laser wavelengths in a fiber amplifier, etc.
However, the wavelength at which silica optical fibers exhibit a minimum attenuation and which is suitable for long-haul communication systems is 1.55 xcexcm. Therefore, it is necessary in order to use an optical fiber diffraction grating at this wavelength that the grating spacing should be about 500 nm. At the beginning, it was deemed to be difficult to make such a fine structure in the core. To make a Bragg diffraction grating in the core of an optical fiber, the conventional practice is to carry out many complicated process steps including side polishing, photoresist process, holography exposure, reactive ion beam etching, etc. For this reason, the conventional practice suffers a long production time and a low yield.
However, there has recently been known a method of making a diffraction grating by irradiating an optical fiber with ultraviolet radiation to produce a refractive index change directly in the core of the optical fiber. The method using irradiation with ultraviolet radiation needs no complicated process. Therefore, increasing use has been made of the method using ultraviolet radiation with the progress of peripheral techniques.
With the method using ultraviolet radiation, because the grating spacing is as fine as about 500 nm, as has been stated above, the following methods are adopted: an interference method wherein two light beams are caused to interfere with each other; a method wherein writing is carried out for each point (single pulses from an excimer laser are focused to form diffraction grating surfaces one by one); a method wherein irradiation is effected by using a phase mask having a grating; and so forth.
The above-described interference method in which two light beams are caused to interfere with each other involves problems in terms of the quality of lateral beams, i.e. spatial coherence. The method wherein writing is carried out for each point needs precise step control of the order of submicrons and requires writing many surfaces with light narrowed down. Therefore, the method suffers from a problem in terms of operability.
Accordingly, the irradiation method using a phase mask has attracted attention as a method capable of coping with the above-described problems. This method uses, as shown in FIG. 5(a), a phase shift mask 21 having grooves with a predetermined depth that are provided on one surface of a quartz substrate at a predetermined pitch. A KrF excimer laser beam (wavelength: 190 to 300 nm) 23 is applied to the mask 21 to produce a refractive index change directly in a core 22A of an optical fiber 22, thereby fabricating a grating (reference numeral 22B denotes a cladding of the optical fiber 22). It should be noted that in FIG. 5(a) an interference fringe pattern 24 in the core 22A is shown as enlarged so as to be readily understandable. FIG. 5(b) is a sectional view of the phase shift mask 21. FIG. 5(c) is a fragmentary top view corresponding to the sectional view. The phase shift mask 21 has a binary phase type diffraction grating structure in which grooves 26 with a depth D are provided on one surface of the phase shift mask 21 at a repeating pitch P, and a strip 27 with approximately the same width as that of each groove 26 is provided between each pair of adjacent grooves 26.
The depth D of the grooves 26 in the phase shift mask 21 (i.e. the height difference between the strips 27 and the grooves 26) is selected so that the phase of the excimer laser beam 23, which is exposure light, is modulated by a xcfx80 radian. Zeroth-order light (beam) 25A is reduced to 5% or less by the phase shift mask 21. Principal light (beam) emerging from the mask 21 is separated into plus 1st-order diffracted light 25B, which includes 35% or more of the diffracted light, and minus 1st-order diffracted light 25C. Thus, the optical fiber 22 is irradiated with interference fringes of predetermined pitch determined by the plus 1st-order diffracted light 25B and the minus 1st-order diffracted light 25C, thereby producing a refractive index change at this pitch in the optical fiber 22.
The grating fabricated in the optical fiber by using the above-described phase mask 21 has a uniform pitch. Therefore, the grooves 26 of the phase mask 21 used to fabricate the grating also have a uniform pitch.
To make such a phase mask, a quartz substrate coated with an electron beam resist is irradiated with an electron beam at regions corresponding to the grooves 26 by an electron beam writing system, and the irradiated regions are removed by etching.
Incidentally, there has recently been a demand for a chirped grating as a Bragg diffraction grating to be formed in optical fibers. The chirped grating is such a diffraction grating that the grating pitch increases or decreases linearly or nonlinearly according to the position in a direction (repeating direction of the grating) perpendicular to the grating grooves. Such a grating is used, for example, as a means for compensating for chromatic dispersion of a high-reflecting mirror having a widened reflection band or that of an optical fiber.
To make a grating in which the grating pitch changes linearly or nonlinearly according to the position in the longitudinal direction of the optical fiber, as stated above, on the basis of the interference of plus 1st-order diffracted light and minus 1st-order diffracted light by using a phase mask, it is necessary that the pitch of the grooves of the phase mask should increase or decrease linearly or nonlinearly according to the position as well, as will be clear from the principle shown in FIG. 5(a) (as the pitch of the grooves of the phase mask decreases, the angle formed between the plus 1st-order diffracted light and the minus 1st-order diffracted light increases, and the pitch of the interference fringes decreases). To make such a phase mask by writing using an electron beam writing system, the conventional practice needs a large amount of writing data for writing grooves or strips between them over the whole area of the mask. Accordingly, there are cases where it is difficult to produce the phase mask. The writing data may have errors occurring in relation to the address grid.
In fabricating a grating in which the grating pitch changes linearly or nonlinearly, the problem of pitch deviation (connection error) may arise at the joint between patterns different in grating pitch from each other. An optical fiber diffraction grating fabricated by using a phase mask containing such connection errors produces a large number of unwanted peaks other than the desired peak of the spectrum, as shown in FIG. 16, by way of example, which illustrates reflection characteristics. In a chirped grating, the connection errors result in ripples in the group delay characteristics. This may give rise to a serious problem when the chirped grating is used to compensate for dispersion of the optical fiber.
The present invention was made in view of the above-described problems with the prior art. An object of the present invention is to provide a method of producing an optical fiber-processing phase mask having minimized connection errors that may degrade the spectral line shape and group delay characteristics of an optical fiber diffraction grating fabricated by using the phase mask. The present invention also includes an optical fiber with a Bragg diffraction grating fabricated by using such an optical fiber-processing phase mask.
To attain the above-described object, the present invention provides a method of producing an optical fiber-processing phase mask having a repeating pattern of grating-shaped grooves and strips provided on one surface of a transparent substrate, so that diffracted light produced by the repeating pattern is applied to an optical fiber to fabricate a diffraction grating in the optical fiber by interference fringes of diffracted light of different orders. In making a mask having a plurality of juxtaposed patterns having a linearly or nonlinearly increasing or decreasing pitch and a uniform groove-strip width ratio, multiple exposure is carried out to minimize difference between the pitch at the joint between patterns having different pitch data and the pitch in each individual pattern.
In this case, when the plurality of patterns having different pitch data are written in juxtaposition with each other by multiple exposure, multiple writing operations may be carried out in the same direction. Alternatively, the multiple writing operations may be carried out in opposite directions.
The pitch of the repeating pattern of grating-shaped grooves and strips is set so as to vary usually between 0.85 xcexcm and 1.25 xcexcm in order to reflect light in the near infrared region.
It is desirable that the difference in height between the grooves and strips of the repeating pattern of grating-shaped grooves and strips should be of such a magnitude that a phase shift of approximately xcfx80 occurs when ultraviolet radiation for processing the optical fiber passes through the phase mask.
The arrangement may be such that the repeating pattern of grating-shaped grooves and strips is based on writing data concerning a basic pattern consisting of one groove and one strip, and the patterns of grooves and strips having different pitches are continuously written by using the writing data concerning the basic pattern while varying the reduced scale for the writing data.
In this case, it is desirable that a change in pitch according to position of the repeating pattern of grating-shaped grooves and strips should be determined according to a change in pitch of the diffraction grating to be fabricated in the optical fiber and should be given by a change according to the reduced scale for the writing data concerning the basic pattern.
It should be noted that writing may be performed by an electron beam writing system or a laser beam writing system.
The present invention also includes an optical fiber with a Bragg diffraction grating fabricated by using an optical fiber-processing phase mask produced by any one of the above-described production methods.
The optical fiber with a Bragg diffraction grating is used to compensate for dispersion of the optical fiber, for example. The group delay ripple of the optical fiber with a Bragg diffraction grating is within xc2x110 ps.
In the present invention, multiple exposure is carried out to make a mask having a plurality of juxtaposed patterns with a uniform groove-strip width ratio. Therefore, positional inaccuracies are averaged as shown in FIG. 1, so that the connection error at the joint between patterns having different pitches decreases. Accordingly, it is possible to minimize connection errors occurring when patterns having different pitches are connected in juxtaposition with each other as in the prior art. In an optical fiber with a Bragg diffraction grating fabricated by using such a phase mask, unwanted peaks other than the desired peak of the spectrum are reduced. In addition, ripples in the group delay characteristics are reduced.
The production method according to the present invention is useful for both a process in which patterns are written with the reduced scale factor being varied, and a process in which repeating pattern data of grating-shaped grooves and strips, which have different pitches, are written in juxtaposition with each other.